Cross Flow Filter Device With Concentric Filter Elements

ABSTRACT

Cross flow filter device comprising a longitudinal first filter element ( 1 ) having a first width (w 1 ) and a longitudinal central axis ( 8 ), a longitudinal second filter element ( 2 ) having a second width (w 2 ) and being longitudinally installed substantially concentrically inside said first filter element ( 1 ), a housing ( 3 ) surrounding said first filter element (1), said cross flow filter device having respectively a raw material inlet ( 4 ), a concentrate outlet ( 5 ), and a first filtrate outlet ( 6 ) connected to said first filter element ( 1 ). The ratio between the second width (w 2 ) and the first width (w 1 ) is 65%, preferably at least 70% and more preferably about 75%.

The present invention relates to a cross flow filter device provided forfiltering a raw material comprising particles in a medium, said crossflow filter device comprising:

-   -   a longitudinal first filter element having a first width and a        longitudinal central axis,    -   a longitudinal second filter element having a second width,        smaller than said first width, and being longitudinally        installed substantially concentrically inside said first filter        element,    -   a housing surrounding said first filter element,        said cross flow filter device having respectively a raw material        inlet, a concentrate outlet provided to exit a concentrate being        said raw material substantially enriched in particles, and a        first filtrate outlet provided to exit a filtrate being said        medium substantially depleted in particles, said filtrate outlet        being connected to said first filter element.

In the art of filter devices, several types of filter devices are known.The two major types of filter devices are the dead end filter device andthe cross flow filter device.

Traditionally, filter devices are used to filter a raw materialcomprising particles in suspension in a medium which can be liquid orgaseous. After filtration, or ultrafiltration, depending of the cut-offof the filter elements, and as aforementioned, a filtrate being the rawmaterial depleted in particles, a concentrate being the raw materialenriched in particles are both obtained. Depending on the application,the product of interest is either the filtrate or the concentrate.

The most important difference between these two types of filter devices,is as follows:

-   -   the dead end filter device comprises a raw material inlet and a        filtrate outlet, the concentrate being generally maintained        inside the device,    -   the cross flow filter device comprises a raw material inlet, a        filtrate outlet and a concentrate outlet.

A dead end filter device is for example known from DE 38 05 361 whichdiscloses several embodiments of the dead end filter device. The deviceaccording to DE 38 05 361 comprises for example two or more concentricfilter elements, an external compartment being between the housing ofthe device and the most external filter element and a centralcompartment, being inside the most internal concentric filter elements.The device also comprises a raw material inlet and a filtrate outlet.The raw material inlet is in direct communication with the externalcompartment and the filtrate outlet is in direct communication with thecentral compartment. The filter elements are provided for retainingparticles with a predetermined particle size and to allow the passage ofthe particles which present a particle size smaller than thepredetermined particle size through the filter element. Therefore, theraw material comprising particles in suspension in a medium enters thefilter device in the external compartment, the particles having aparticle size greater than the predetermined particle size are retainedupstream and the medium with the particles having a particle sizesmaller than the predetermined particle size passes through the filterelement. The same applies to all concentric filter elements.

Thus, the particles having a size greater than the predeterminedparticle size of the filter element downstream and a size smaller thanthe predetermined particle size of the filter element upstream areretained between two filter elements. Further, the filter devicecomprises a cleaning device to harvest or to remove the concentrate andthe particles being retained between each filter element.

The cross flow filter devices always comprise a concentrate outlet toharvest or to remove the concentrate and the particles. But generally, acleaning device is still needed because of particle deposition andclogging of the filter element.

Such a cross-flow filter device, as mentioned in the preamble of claim1, is known, for example, from DE 197 03 877 which describes an assemblyto clean a filter membrane of a cross flow filter. A layer ofsedimentary deposits is rinsed off by fluid directed at the depositsfrom a nozzle a short distance away. In the filter device according toDE 197 03 877, there is a concentric positioning of multiple filtermembranes with sufficient space between the membranes for having insidecleaning nozzles that clean the surface of the filter media like a jetwasher.

Generally, and as it is the case in DE 197 03 877 (see drawings), theratio between the second and the first filter membrane width is around0.55.

Unfortunately, such a cross flow filter device is very difficult tomanufacture and to operate and does not prevent or reduce theaccumulation of sediments at the surfaces of the filter element.

It is an object of the invention to palliate at least some of thesedrawbacks by providing a cross flow filter device which is easy tomanufacture, easy to carry out while providing better performance byimproving the filtration effect due to a reduction of the membraneclogging and particle deposition and an increase of the filtrationsurface of a given space requirement.

To this end, the invention provides a cross flow filter device accordingto the preamble of claim 1, characterised in that the ratio between thesecond width and the first width is 65%, preferably at least 70% andmore preferably about 75%.

It has been surprisingly found that the ratio between the second widthand the first width of at least 65% substantially reduced the cloggingand the particle deposition. Indeed, the presence of such a little spacebetween the two filter elements involves an increased velocity for rawmaterial while substantially reducing or preventing particle depositionor membrane clogging. Indeed at a constant flow rate of the rawmaterial, if the space between the two filter elements is reduced, thevelocity of the raw material will increase. Thus it is desirable toreduce the space between the two filter elements by adjusting the ratiobetween the second width and the first width to at least 65%, preferablyto least 70% and more preferably to about 75% to reduce or preventpermanent particle deposition and membrane clogging without requiringany cleaning device between the two filter elements (like jet washers,scrapers or the like).

Preferably, the ratio between the second width and the first width is atmost 95% for allowing a predetermined flow rate of the raw material andto avoid an overpressure effect at the raw material inlet.

Another advantage of the device according to the invention is that thefirst filter element and the second filter element are two separatefilter elements having each their own filtrate outlet being differentfrom each other, the filtrate outlet of the first filter element beingsaid first filtrate outlet and the filtrate outlet of the second filterelement being a second filtrate outlet.

With two different filtrate outlets, the device according to theinvention is a more flexible device allowing to use the device with onlyone filter element when required by blanking the outlet of the concernedfilter element, for example, when the concerned filter element isaccidentally perforated.

Advantageously, the raw material inlet and the concentrate outlet aresubstantially aligned, in particular, aligned with said longitudinalcentral axis.

It is advantageous that the raw material inlet and the concentrateoutlet are aligned with the longitudinal central axis as this is thedirection of the raw material flow. Such configuration allows the flowto remain a substantially laminar flow without substantial perturbationand the raw material (concentrate) passes with a higher efficiencytangentially to the surface of the filter element.

In a particular embodiment, the first and the second filtrate outletsare respectively prolonged by a first and a second nozzle which areended respectively by a first and a second valves, each valve having atleast an open position and a closed position and being separatelycontrolled.

This is also a characteristic which allows a more flexible device asmentioned before. When the filtrate outlets present a nozzle ended by avalve, when it is required, it is possible, to blank an outlet. Onevalve being optionally controlled independently from the other or bothvalves being controlled together. Moreover, one filter element can be inoperation while the other could be in a cleaning cycle. Indeed, byhaving two concentric filter elements, they present different diametersimplying that the surface of the first filter element is different thanthe surface of the second element and the two filter elements shouldthen be in a cleaning cycle at different moment. Indeed, with twodifferent surfaces, since each filter element can manage a certainquantity of particles, they do not necessarily need to be cleaned at thesame moment.

Preferably, each filter element comprises a filter media, each filtermedia of each filter element having its own cut-off, said cut-off of thefilter media of the first filter element being substantially differentor similar from said cut-off of the filter media of the second filterelement.

The device according to the invention allows to collect two qualities offiltrates as the cut-off of the first filter media can be different ofthe second filter media. The device according to the invention comprisesin an advantageous embodiment, two independent filtrate outlets allowingto collect the two filtrates with respectively their own qualities.

It can be preferred to have, depending on the application to which thedevice is dedicated, the same cut-off for the two filter elements.

Moreover, each filter media being manufactured in a material selected inthe group consisting of metallic material, organic material or inorganicmaterial.

The material of the filter media can be the same for the two filterelements or different, such material is chosen according to the chemicaland physical properties of the raw material, to the compatibility orincompatibility between the raw material and the filter media and alsoaccording to the porosity. The yield of the device for a givenapplication is also taken into account for the choice of the materialforming the filter media.

The device according to the invention comprises, advantageously, acirculation pump which is provided between the concentrate outlet andthe raw material inlet.

There could be a tank downstream and upstream to store raw materialrespectively not passed and passed one time through the filtrationdevice, but advantageously, the raw material is recirculated through thedevice either to concentrate the raw material or to reduce losses,namely if what should be collected is very small particles passingthrough the filtration media, and having to be removed from the rawmaterial, as explained in more details hereinafter.

Moreover, in a particular embodiment, a back-flush device is providedcomprising:

-   -   an expansion vessel, having a first port and a second port,        identical or different from said first port,    -   a third nozzle connected to said first port of the expansion        vessel and to said first nozzle between the first filtrate        outlet and the first valve, and    -   a fourth nozzle connected to said second port of the expansion        vessel and said to said second nozzle between the second        filtrate outlet and the second valve.

Generally, most common back-flush device comprises a buffer vessel withfiltrate which is pressurised with compressed air or another gas or apump situated after the filter elements that pumps the filtrate back inreverse direction.

Common back-flush device using pressurisation are discontinuous devicewhich does not allow a continuous process to be carried out therein,they are complicated and require an heavy maintenance.

Here, the invention comprises an expansion vessel which does not needany pressurised gas nor additional pump at the filtrate outlet so thedesign and the maintenance of the system is simplified.

In a particularly advantageous embodiment, the third nozzles comprises athird valve and the fourth nozzle comprises a fourth valve, said thirdand said fourth valves having each at least an open position and aclosed position and being separately controlled.

When the third valve is in open position, the filtrate normallycollected by the first nozzle can fill the expansion vessel. When thefourth valve is in open position, the filtrate normally collected by thesecond nozzle can fill the expansion vessel. The third and the fourthvalves are separately controlled, so it could be the third or the fourthor both which are in open position. When the level of filtrate in theexpansion vessel is sufficient, the valves (third, fourth or both) inopen position have to be closed. When the surface of the first or thesecond filter element is clogged or shows deposits, respectively thethird or the fourth valve, will be opened to clean the surface of thefilter element.

Because the expansion vessel is connected to the two filtrate outlets,and because the valves are separately controlled, when a surface has tobe cleaned, the first filter element of the cross flow filter device canbe in a cleaning cycle when the second is in use or inversely.

Moreover, the concentrate outlet is ended by a valve, in particular by athrottle valve. The valve of the concentrate outlet allow to transformthe cross flow filter device into a dead end device by simply closingthe throttle valve. Moreover, the throttle valve allows to build uppressure as the expansion vessel is pressurised using the filtrate flow.The pressure of the filtrate flow is increased by throttling the rawmaterial concentrate so that the pressure in the filter module is builtup. This pressure will go through the filter element to the filtrateside of the module and thus, it is possible to use this pressure topressurise the expansion vessel.

Once the expansion vessel has reached its maximal pressure, the thirdand the fourth valves were closed and the throttle valve is opened sothat the cross flow effect is recovered in the cross flow filtrationdevice according to the invention.

Other embodiments of the filtration device according to the inventionare mentioned in the annexed claims.

Other characteristics and advantages of the invention will appear moreclearly in the light of the following description of a particularnon-limiting embodiment of the invention, while referring to thefigures.

FIG. 1 is a cross section of the cross flow filter device according tothe invention

FIG. 2 is a cross section of the cross flow filter device according tothe invention further comprising a circulation pump, outlet valves andinlet valves.

FIG. 3 is a cross section of the cross flow filter device according tothe invention integrated in a filtration device further comprising aback-flush device, a circulation pump and a concentrate outlet throttlevalve.

FIG. 4 a is a cross section of the expansion vessel of the back-flushdevice of the filtration device without filtrate and 4 b is the samerepresentation of the expansion vessel but full of filtrate.

FIG. 5 is a cross section of the filtration device showing the fillingand the pressurising of the expansion vessel with filtrate.

FIG. 6 is a cross section of the filtration device showing theback-flushing of the first filter element to remove deposition ofparticles while the second filter element is still in operation.

FIG. 7 is a cross section of the filtration device showing theback-flushing of the second filter element to remove deposition ofparticles while the first filter element is still in operation.

FIG. 8 is a cross section of the filtration device showing theback-flushing of the first and of the second filter elements to removedeposition of particles. Circulation of raw material is maintained tocarry away the removed deposition into concentrate flow.

In the drawings, a same reference sign has been allotted to a same oranalogous element of the cross flow filtration device according to theinvention.

In FIG. 1, it can be seen that the device according to the inventioncomprises a longitudinal first filter element 1 and a longitudinalsecond filter element 2 installed substantially concentrically insidethe first filter element 1. The cross flow filter device also comprisesa housing 3 surrounding the first filter element 1, a raw material inlet4, a concentrate outlet 5, and a first filtrate outlet 6 connected tothe first filter element 1. As shown, the raw material inlet 4 and theconcentrate outlet 5 are preferably substantially aligned, inparticular, aligned with said longitudinal central axis 8. In thispreferred embodiment, the second filter element 2 is connected to itsown filtrate outlet called the second filtrate outlet 7.

Each filter element 1,2 comprises a filter media having its own cut-off.The cut-off of the filter media of the first filter element 1 can besubstantially similar or different from said cut-off of the filter mediaof the second filter element 2. For example, the cut-off of the firstfilter media can range from 500 to 0.1 μm, preferably from 100 to 1 μm.

The filter media, can be manufactured, for example, in a materialselected in the group consisting in metallic material, organic materialor inorganic material. Exemplary materials are stainless steel,ceramics, polyethersulfone, polypropylene and the like.

As illustrated in FIG. 2, the first 6 and the second filtrate outlets 7are respectively extended by a first 9 and a second nozzle 10. The firstnozzle 9 is ended by a first valve 11 and the second nozzle 10 is endedby a second valve 12.

Moreover, in a preferred embodiment, a circulation pump 13 is providedbetween the concentrate outlet 5 and the raw material inlet 4.

The raw material to be filtered is pumped from a process plant or from araw material tank to the cross flow filter device.

It should be intended that it can be the circulation pump 13 or anotherpump which supplies the raw material to be filtered to the cross flowfilter device. Indeed, the circulation pump 13 can make the connectionbetween the raw material tank and the concentrate outlet 5 beforesupplying a mixture of the raw material and of the concentrate at theinlet 4 of the cross flow filter device.

According to the invention, the raw material inlet or the mixtureaforementioned is supplied tangentially to the filter media surface ofthe filter element as it is indicated by arrows.

The first filter element has a first width w1 and the second filterelement has a second width w2. The first width w1 is the inner width ofthe first filter, whereas the second width is the outer width of thesecond filter. To improve the performance of the cross flow filtrationdevice, the ratio between the second width and the first width is 65%,preferably at least 70% and more preferably about 75%.

With such values, the cleaning effect of the surfaces of both filterelements by the raw material flow is increased. Indeed, by reducing thevolume easily accessible for the raw material into the cross flow filterdevice, the inside pressure is increased. (Bernoulli: P.V=constant).

Therefore, as the raw material inlet flow rate is defined by thecirculation pump, it is relatively constant and with respect to what wassaid before, the velocity of the raw material is increased. As aconsequence, high cross flow velocity tangentially to the filter mediasurface reduces or prevents a permanent particle deposition and thus aclogging of membranes pores.

In a similar way, the ratio between w2 and w1 is preferably at most 95%in order to allow the raw material to enter in the cross flow filterdevice according to the invention at an appropriate flow rate. Thisratio also avoids an overpressure at the raw material inlet.

In this particularly preferred embodiment, the second filter element 2is connected to its own filtrate outlet called the second filtrateoutlet 7. The second filtrate outlet 7 is connected to a second nozzle10 and the second nozzle 10 comprises a second valve 12. The firstfiltrate outlet 6 is also connected to a first nozzle 9 and comprises afirst valve 11. Both valves 11,12 are preferably separately controlledallowing to operate the two filter elements 1,2 independently. The firstfilter element 1 can be in a cleaning cycle when the second 2 is infiltration operation.

It should be understood that the alignment of the raw material inlet 4and the concentrate outlet 5 is important as it contributes totangentially apply the raw material flow to the filter media surfacewithout causing turbulence which can damage the rather inside laminarflow.

The maximal value of the ratio of 95% also enables to avoid turbulencesin the raw material flow.

Preferably, the two filter elements 1,2 have the same removal efficiencyby having a similar cut-off. But when two different qualities ofproducts are needed, it can be advantageous to have a different cut-offfor the filter media on each filter element.

The first 11 and the second valves 12 of the filtrate outlets 6, 7 canbe directly connected by nozzles to another process plant for immediateuse or to one or two filtrate tanks for storage. If the filter media ofthe filter elements are similar, the filtrate outlets of the cross flowfilter device according to the invention can be connected to one or twofiltrate tanks to store the filtrate. If the cut-off is different forthe two filter media, it should be understood that two different tanksshould be provided to store the two different filtrates as the qualityand the composition is not the same.

In summary, by the presence of the second filter element 2, the filtersurface in the filtration device is increased. This latter acts as areducer, increasing the velocity of the raw material in the filtrationdevice and thus improving the cleaning effect of the tangential flowover the filter media, resulting in longer operating cycles beforecleaning procedures. Moreover, the two filter elements 1,2 can beoperated individually allowing a continuous operation of the filtrationdevice.

Exemplary values for dimensions of the elements of the cross flow filterdevice are as stated in the following table (Table 1). TABLE 1 Innervolume of the first filter element 453 cm³ Outer volume of the secondfilter element 245 cm³ Volume between both filter elements 208 cm³ Widthw1 of the first filter element 33 mm Width w2 of the second filterelement 25 mm Number of times the velocity increased 2.17 times

When in service, the raw material is fed by the circulation pump 13through the raw material inlet 4 into the space between the two filterelement 1,2. Depending on the cut-off of the filter media of the filterelements 1,2, the fluid (liquid or gas) passes through the filterelements 1,2, which fluid contains several particles which are smallerthan the size of the filter media pores. Greater particles remain intothe space between both filter element 1,2. A portion of the particleswill be deposited upon the surface of the filter media and the otherportion will be carried away by the flow. This is the reason why theoutlet of raw material is called concentrate outlet 5 as the fluid isenriched with particles.

The fluid which has passed through the filter elements 1,2 exits via thefirst 6 and the second filtrate outlet 7 and is either directed toanother process plant or to a filtrate tank.

FIG. 3 shows a cross section of the cross flow filter device accordingto the invention integrated in a filtration device further comprising aback-flush device. The back-flush device comprises an expansion vessel17 having a first port 14 (inlet) which is also the second port(outlet). A third nozzle 15 is connected to the first port 14 of theexpansion vessel 17 and to said first nozzle 9 between the firstfiltrate outlet 6 and the first valve 11, and a fourth nozzle 16 isconnected to the port 14 of the expansion vessel 17 and to the secondnozzle 10 between the second filtrate outlet 7 and the second valve 12.

The third nozzle 15 comprises a third valve 24 between the first nozzle9 and the port 14 of the expansion vessel. The fourth nozzle 16comprises a fourth valve 25 between the second nozzle 10 and the port14.

The first 11 and the second valves 12 of the filtrate outlets 6, 7 canbe directly connected by nozzles to another process plant for immediateuse or to one or two filtrate tanks for storage.

As aforementioned, the raw material to be filtered is fed by acirculation pump 13 from a process plant or from a raw material tank tothe filtration device.

As also explained before, it should be intended that it can be thecirculation pump 13 or another pump which supplies the raw material tobe filtered to the filtration device. Indeed, the circulation pump 13can make the connection between the raw material tank and theconcentrate outlet 5 before supplying a mixture of the raw material andof the concentrate at the inlet 4 of the filtration device.

Moreover, the concentrate outlet 5 is ended by a fifth valve 23, beingin particular a throttle valve. The throttle valve is provided toregulate the flow rate of the concentrate by throttling this later.

FIG. 4 shows details of the expansion vessel of the back-flush deviceaccording to the invention which can be used in the filtration device.

FIG. 4 a shows the expansion vessel without filtrate and 4 b is the samerepresentation of the expansion vessel but full of filtrate. Theexpansion vessel is, in particular, an expansion vessel similar to thoseused in heating systems.

The expansion vessel comprises a housing 18, preferably made ofstainless steel, a diaphragm 19 dividing the vessel in two parts, anexternal first part 20 provided to contain a gas (being in factcontained out of the diaphragm 19 and within the housing 18) and ainternal second part 21 provided to contain a liquid (being the interiorof the diaphragm 19). The diaphragm 19 is preferably interchangeable andmade of butyl rubber. The material used to manufacture the expansionvessel housing 18 can be any material but preference is given tostainless steel because all component that are not made of this materialcan be damaged by salt or other substances that may optionally becontained in filtrate or in air.

Also the diaphragm 19 can be made of any material well known by thoseskilled in the art, but butyl rubber is preferred for its elasticity,resistance and neutrality. It should be understood that preferably, thematerial either for the expansion vessel housing 18 or the diaphragm 19are chosen to not interact with liquid or gas that will be containedinto the expansion vessel 17.

In this particular embodiment, the expansion vessel further comprises asingle port 14 as inlet and outlet for filtrate since valves are presentto impose the sense of the filtrate (coming in or out). It should beintended that two ports can also be present i.e. an inlet port and anoutlet port without changing anything to the operation of the back-flushdevice.

The port 14 is provided to allow the filtrate coming from the cross flowfilter device to fill through the diaphragm 19 the internal first part21 of the expansion vessel 17 which is provided to contain the filtrate.

An additional valve 22 is provided in the second part 20 to allow excessof gas to go out to avoid the overpressure in the second part of thevessel.

As it can be seen at FIG. 3, and as mentioned before, when in service,the raw material is fed by the circulation pump 13 through the rawmaterial inlet 4 into the space between the two filter elements 1,2. Thefluid (liquid or gas) passes through the filter media of the filterelements 1,2 containing several particles which are smaller than thesize of the filter media pores. Greater particles remain into the spacebetween both filter elements 1,2. A portion of the particles will bedeposited upon the surface of the filter media and the other portionwill be carried out by the flow.

The fluid which has passed through the filter elements exits via thefirst 6 and the second filtrate outlet 7 and is either directed toanother process plant, to a filtrate tank or to the back-flush device,depending on the valve positions. It should be considered that afiltrate tank and the back-flush device can be fed together by thefiltration device according to the invention. The operation of thefiltration device using the back-flush device is explained hereinafterin more details.

As it can be seen in FIG. 4 b, the filtrate is fed in the first part 21which increase in volume with filling. The filling with filtrate of thisarea results in a pressurisation of the second part 20 as the gascontained in the second part 20 is compressed by the increasing volumeof the first part 21. Therefore, the second part 20 exerts also apressure onto the diaphragm 19, which pressure is useful to clean one orboth filter elements 1,2 when back-flush flow is required.

FIG. 5 shows several possibilities for the filling and the pressurisingof the expansion vessel with filtrate. The direction of the filtrateflows is indicated by arrows in the different nozzles.

To build up pressure, the third 24 and the fourth 25 valves should be inopen position and the first 11 and the second valves 12 should be in aclosed position. Therefore, the expansion vessel 17 is pressurised usingthe filtrate flow. By throttling the throttle valve 23 of theconcentrate outlet 4, the pressure of the filtrate flow is increased sothat the pressure builds up in the filtration device.

This pressure will go through the filter elements 1, 2 to the filtrateside of the filtration device and thus, this pressure can be used topressurise the expansion vessel 17 by filing it with the filtrate.

It should be understood that the expansion vessel 17 can be filled onlywith the filtrate coming from the first filter element 1, from thesecond filter element 2 or both.

The following table (Table 2) shows different possible configuration ofthe valves to fill the expansion vessel with filtrate while the arrowsin FIG. 5 show the direction of the filtrate flows during theseoperations. TABLE 2 First valve Second Third Fourth (11) valve (12)valve (24) valve (25) Fill via second filter open close close openelement (2) Fill via first filter close open open close element (1) Fillvia both filter close close open open element (1 and 2)

Once the expansion vessel 17 has reached its maximal pressure, the third24 and/or the fourth valves 25 are closed and the fifth valve 23 isopened by throttling to allow the concentrate outlet so that the crossflow effect comes back in the module.

FIG. 6 shows the back-flushing of the first filter element 1 to removedeposition of particles while the second filter element 2 is still inoperation.

For the following explanation, it should be envisaged that the expansionvessel 17 has reached its maximal pressure, and that the filtrationdevice has been operated between the filling of the expansion vessel andthe cleaning of the first filter element 1.

The following table (Table 3) shows the position of the valves when thefirst filter element 1 is back-flushed while the second filter element 2is in operation. The direction of the filtrate during this operation isindicated by arrows in FIG. 6. TABLE 3 First Second Third Fourth valve(11) valve (12) valve (24) valve (25) Back-Flush of the first close openopen close filter element 1, second filter element 2 in operation

FIG. 7 shows the back-flushing of the second filter element 2 to removedeposition of particles while the first filter element 1 is still inoperation.

The following table (Table 4) shows the position of the valves duringthe back-flushing of the second filter element 2 while the operation ofthe first filter element 1. The direction of the filtrate during thisoperation is indicated by arrows in FIG. 7. TABLE 4 First Second ThirdFourth valve (11) valve (12) valve (24) valve (25) Back-Flush of theopen close close open second filter element 2, first filter element 1 inoperation

FIG. 8 shows the back-flushing of the first 1 and of the second filterelement 2. Circulation of raw material is maintained to carry away theremoved deposition into concentrate flow.

The following table (Table 5) shows the position of the valves duringthe back-flushing of the first 1 and of the second 2 filter elementwhile circulation of the fluid in the space between the two filterelement is maintained to carry away the removed particle into theconcentrate flow.

The direction of the filtrate during this operation is indicated byarrows in FIG. 8. TABLE 5 First Second Third Fourth valve (11) valve(12) valve (24) valve (25) Back-Flush of the close close open open first1 and of the second filter element 2

Although the preferred embodiments of the invention have been disclosedfor illustrative purpose, those skilled in the art will appreciate thatvarious modifications, additions or substitutions are possible, withoutdeparting from the scope and spirit of the invention as disclosed in theaccompanying claims.

For example, the valves of the cross flow filter device according to theinvention or of the back-flush device can be manually or automaticallyclosed and opened.

1. Cross flow filter device provided for filtering a raw materialcomprising particles in a medium, said cross flow filter devicecomprising: a longitudinal first filter element (1) having a first width(w1) and a longitudinal central axis (8), a longitudinal second filterelement (2) having a second width (w2), smaller than said first width,and being longitudinally installed substantially concentrically insidesaid first filter element (1), and a housing (3) surrounding said firstfilter element (1), said cross flow filter device having respectively araw material inlet (4), a concentrate outlet (5) provided to exit aconcentrate being said raw material substantially enriched in particles,and a first filtrate outlet (6) provided to exit a filtrate being saidmedium substantially depleted in particles, said filtrate outlet beingconnected to said first filter element (1), wherein the ratio betweenthe second width (w2) and the first width (w1) is at least 65%.
 2. Crossflow filter device according to claim 1, wherein said ratio between thesecond width (w2) and the first width (w1) is at least 70%.
 3. Crossflow filter device according to claim 1, wherein said ratio between thesecond width (w2) and the first width (w1) is about 75%.
 4. Cross flowfilter device according to claim 1, wherein said ratio between thesecond width (w2) and the first width (w1) is at most 95%.
 5. Cross flowfilter device according to claim 1 wherein the first filter element (1)and the second filter element (2) are two separate filter elements (1,2)having each their own filtrate outlet (6,7) being different from eachother, the filtrate outlet (6) of the first filter element (1) beingsaid first filtrate outlet (6) and the filtrate outlet (7) of the secondfilter element (2) being a second filtrate outlet (7).
 6. Cross flowfilter device according to claim 1, wherein the raw material inlet (4)and the concentrate outlet (5) are substantially aligned with saidlongitudinal central axis (8).
 7. Cross flow filter device according toclaim 1, wherein the first (6) and the second filtrate outlets (7) arerespectively prolonged by a first (9) and a second nozzle (10) which areended respectively by a first (11) and a second valves (12), each valve(11,12) having at least an open position and a closed position and beingseparately controlled.
 8. Cross flow filter device according to claim 1,wherein each filter element (1,2) comprises a filter media, each filtermedia of each filter element (1,2) having its own cut-off, said cut-offof the filter media of the first filter element (1) being substantiallydifferent from said cut-off of the filter media of the second filterelement (2).
 9. Cross flow filter device according to claim 1, whereineach filter element (1,2) comprises a filter media, each filter media ofeach filter element (1,2) having its own cut-off, said cut-off of thefilter media of the first filter element (1) being substantially similarto said cut-off of the filter media of the second filter element (2).10. Cross flow filter device according to claim 9, wherein each filtermedia is manufactured from a material selected from the group consistingfrom metallic material, organic material or inorganic material. 11.Cross flow filter device according to claim 10, including a circulationpump (13) between the concentrate outlet (5) and the raw material inlet(4).
 12. Cross flow filter device according to claim 11, wherein aback-flush device is provided comprising: an expansion vessel (17),having a first port (14) and a second port, identical or different fromsaid first port (14), a third nozzle (15) connected to said first port(14) of the expansion vessel (17) and to said first nozzle (9) betweenthe first filtrate outlet (6) and the first valve (11), and a fourthnozzle (16) connected to said second port of the expansion vessel (17)and to said second nozzle (10) between the second filtrate outlet (7)and the second valve (12).
 13. Cross flow filter device according toclaim 12, in which said third nozzle (15) comprises a third valve (24)and said fourth nozzle (16) comprises a fourth valve (25), said third(24) and said fourth valves (25) having each at least an open positionand a closed position and being separately controlled.
 14. Cross flowfilter device according to claim 13, wherein the concentrate outlet (5)is ended by a throttle valve (23).